In U.S. Pat. No. 3,434,774, it is disclosed that more efficient transmission of optical wave energy along a multimode optical fiber is achieved by grading the refractive index of the fiber core. However, the degree of efficiency that can be derived is determined by how closely the fiber's refractive index profile approaches the optimum distribution. Thus, an accurate knowledge of the profile is necessary in order to assess the fiber's transmission properties.
At present, the most sensitive technique used to obtain fiber profile information is interference microscopy. In application, a very thin transverse sample of the fiber to be tested is prepared and, when placed in one branch of an interferometer, serves as a phase object. Because of its graded-index, the phase shift produced by the core region of the fiber sample is not uniform, resulting in a displacement in the fringe pattern produced by the interferometer. The fringe displacement, or shift, at any point, is proportional to the index difference, .DELTA.n, between the cladding index, which is typically uniform, and the core index at that point. Thus, to obtain the index distribution over the area of the core, the fringe displacement must be carefully measured at a large number of points consistent with the degree of accuracy desired.
The principal problem with this technique is that it is destructive in that a sample of the fiber must be taken. In addition, a very time consuming preparation of the sample is required.
In an alternative method described by D. Gloge and E. A. J. Marcatili in an article entitled "Multimode Theory of Graded-Core Fibers," published in the Bell System Technical Journal of November 1973, pp. 1563-1578, the index profile is derived from the near- and far-field power distribution. However, this technique requires a fixed length of fiber; must be corrected for leaky modes; cannot be used on preforms; and the resulting profile is only an average over the length of the sample.